Cap and pin insulator having an insulating layer to improve radio interference performance



Dec. 16. 1969 c, BULCQCK ET AL 3,484,543

CAP AND PIN INSULATOR HAVING AN INSULATING LAYER TO IMPROVE RADIO INTERFERENCE PERFORMANCE Filed Oct. 14, 1968 EEjI United States Patent 3,484,543 CAP AND PIN INSULATOR HAVING AN INSULAT- ING LAYER TO IMPROVE RADIO INTER- FERENCE PERFORMANCE Clifford James Bulcock, Tamworth, and Terence Edwards, Kingsbury, near Tamworth, England, assignors to Doulton & Co. Limited, London, England, a corporation of the United Kingdom Filed Oct. 14, 1968, Ser. No. 767,406 Claims priority, application Great Britain, Oct. 23, 1967,

48,023/ 67 Int. 'Cl. H01b 17/08 US. Cl. 174-182 2 Claims ABSTRACT OF THE DISCLOSURE A porcelain disc insulator of the cap and pin type having improved radio interference performance provided by a layer of an insulating material, preferably a sand-sulphur-graphite composition which is applied across the mouth of the pin hole so as to be in intimate contact both with the porcelain at the mouth of the pin hole and with the pin itself.

This invention relates to cap and pin disc insulators and is particularly concerned with the problem of radio interference caused by the generation of high frequency voltages at points of high electric stress.

Since in any insulator all the desired properties of mechanical strength, puncture strength, flashover voltage, creepage distance and overall string length, have to be made to specification, there are limitations on the changes which can be made to the basic design of an insulator. Varying the size of the pin hole or the thickness of the porcelain in the head of the insulator can result in some improvement in the radio interference performance but would be accompanied by a reduction in mechanical performance or an increase in the weight and cost of the insulator.

The present invention seeks to improve the radio interference performance of a disc insulator without creating any disadvantages or detrimental effects, this performance being assessed by applying a specified power frequency voltage and measuring the radio noise generated at a frequency of 1 mc./s. The measurements are carried out using a measuring set conforming to British Standard 727 or International Special Committee on Radio Interference specification number 1.

According to the present invention, in a porcelain disc insulator of the cap and pin type, wherein the pin is secured in the pin hole by means of a cement, there is provided a layer of an insulating composition adhering intimately to the porcelain at the mouth of the pin hole and in intimate contact with the pin.

The properties required of such an insulating composition are:

(a) It should have a high dielectric strength (b) It should be easy to apply and convenient to use (c) It should be cheap (d) It must flow well (e) It must stick to porcelain (f) It must not be affected by weather (g) It must not track or tree when subjected to a high electric field.

A preferred composition comprises 20-80 weight percent sulphur, 19.99-69.99 weight percent sand and 001-5 weight percent colloidal graphite, the particle size distribution of the sand lying within the range Patented Dec. 16, 1969 "ice British Standard Percent by wt. of sieve No.2 Designated sieve No. +60 -0 +85 40-0 +120 5-0 120+300 4-0 -300 A: l-

and more particularly in the range Percent by wt.

Sulphur 50-60 Sand 39.5-49.5 Graphite 0.5

Sand is a suitable filler for sulphur as it meets all the requirements listed above and furthermore prevents the contraction which occurs when pure sulphur changes from liquid to solid. The graphite serves as a stabilizer when the composition is in a molten state prior to pouring into the cavity around the insulator pin. It has been found that the particle size distribution of the sand is quite critical in that the larger the grain size the more easily will the sand settle out from suspension in molten sulphur and therefore more graphite is needed to prevent this happening. It has been found that sand within the following British Standard sieve sizes can be used:

British Standard Parts by wt. of

sieve N0.: Designated sieve No. +60s 50-0 -60+85 40-0 85 5-0 -120+300 4-0 300 1-100 The graphites which are suitable for use in this invention can be classed as colloidal graphites, which are of sub-micron size and have no particular shape.

It is also envisaged that, instead of sulphur, other insulating materials such as epoxy resin, bitumen or wax might be used but although improvements in radio interference performance can be obtained, the alternative materials are all less satisfactory from the point of view of the other reqirements listed above. For example, resinous materials are in general satisfactory but not cheap even when used with fillers and they are poor for weather resistance and tracking under high electric fields.

Bitumastic materials tend to flow over a period of time and high melting point bitumastic chilled as it was poured and did not flow well. The addition of thinners to cure this tendency resulted in variable radio interference performance.

In the accompanying drawing an embodiment of the present invention is illustrated. This is a conventional cap and pin type insulator comprising a glazed porcelain body 1 having a pin 2 and metal cap 3. A resilient pad 4 is sandwiched between the top of the pin 2 and the porcelain body 1. The cap 3 carries a phosphor bronze security clip 5.

The cap 3, in accordance with conventional practice, is cemented, with Portland cement based mixture, on the body 1 and likewise Portland cement based mixture is used to anchor the pin 2 in its pin hole.

A layer 6 of insulating material comprising a 50% sulphur, 49 5% sand, 0.5% graphite composition (all percentages being by weight) covers the exposed area of cement which anchors the pin 2 in its pin hole. The sand contains a maximum of 5% material retained on a 60 mesh B.S. sieve. The material of the layer 6 adheres to the porcelain at the mouth of the pin hole and is in intima e contact with the pin 2.

The layer 6 is formed by pouring the sulphur, graphite, sand composition, the sulphur being in a mo ten state,

on to the exposed surface of the cement and allowing the sulphur to cool and set.

During this pouring the insulator will of course be in the inverted position. The presence of the sand as a filler prevents the sulphur from contracting away from the porcelain wall of the pin hole, during cooling, and the graphite acts to stabilize the sand suspension in the mOlten sulphur.

By using the sulphur-sand-graphite composition described above, the applied voltage for a given interference level can be increased by 20% for a given insulator. Thus a string insulator assembly composed of a plurality of units as shown in the accompanying drawing, and intended for use at 400 kv., withstood an increase in test voltage of 60 kv. whilst meeting the radio interference requirements of a specification.

It should perhaps be mentioned that the explanation for the high electrical resistivity of these mixtures containing graphite is that the film loading of the graphite is so low that the graphite particle to particle contact does not occur. Usually about 20-25% by volume of graphite is needed before such a stage is reached, the actual value depending on the particle size and distribution.

What we claim is:

1. In a porcelain disc insulator of the cap and pin type, wherein the pin is secured in the pin hole by means of a cement, a layer of an insulating composition adhering intimately to the porcelain at the mouth of the pin hole and in intimate contact with the pin, said insulating composition comprising 20-80 weight percent of material taken from the group consisting of sulphur, bitumen, a wax and an epoxy resin, 19.99-69.99 weight .1 percent sand and 001-5 weight percent colloidal graphite, the sand having a size distribution lying within the range Percent of wt. des- Mesh British standard sieve No.: ignated sieve No.

2. In a porcelain disc insulator of the cap and pin type, a layer of an insulating composition according to claim 1 comprising 50-60 Weight percent sulphur, 39.5-49.5 weight percent sand, and 0.5 weight percent colloidal graphite.

References Cited UNITED STATES PATENTS LARAMIE E. ASKIN, Primary Examiner US. Cl. X.R. 174-196 

